The Fashion Star mentor, 32, looked radiant on her way in to a pal's birthday dinner Saturday night, showing off lean, toned legs in a super short, sparkly Boyod dress. (By her side for her night out? Fiance Eric Johnson.)

At the celebration -- which took place at L.A.'s Fig & Olive restaurant -- Simpson took a night off from her diet and indulged in Fig & Olive salad, filet mignon, and the Chocolate Pot de Creme for dessert. (She also sipped on a Bellini and took part in a champagne toast honoring the birthday girl.)

According to an eyewitness, new parents Simpson and Johnson, who welcomed daughter Maxwell May 1, were set on making the most of their night out together.

Even though there were more than 20 guests at the get-together, "They were whispering into each other's ears and smiling and laughing a lot," the witness tells Us Weekly. "They spent the majority of the evening engaged in each other's company."